Subterranean reservoir rock formations that contain hydrocarbons and gases are often, if not usually, horizontal in profile. It was therefore of immense economic value and a great benefit to society when modern drilling techniques were developed that could create horizontal wellbores from a vertical well over a distance to gain access to a larger portion of hydrocarbon and natural gas resources in a reservoir.
A problem to overcome, however, was that such horizontal reservoirs (for instance, shale formations), are generally quite tight and compressed in nature, meaning that they often don't contain natural fractures of sufficient porosity and permeability within the formation through which hydrocarbons and gas can readily flow into the well at economic rates. Engineers, however, were able to develop methodologies whereby rock formations can be “perfed” (perforated) and “fracked” (fractured) to create pathways in the rock formations through which hydrocarbons and gas can much more readily flow to the well.
While such fracking has led to a great increase in the amount of hydrocarbons and gas that can be readily recovered from a formation, engineers found that it was important to be able to isolate one fracture from another so that the same part of the well was not being repeatedly fractured. Repeated fracturing can cause rock chips and fine rock particles to enter cracks and pore space, thereby reducing the porosity and permeability of the fracked area into the well. The same is true for vertical or deviated wells.
In the known methodology, drilling, and perfing and fracking rock formations involves separate operations. In particular, the well is drilled first, and then the drilling rig is moved off location before a fracturing “spread” is moved on to the location to perf and frac the wellbore for the subsequent recovery of hydrocarbon or natural gas resources. The timing between the drilling of the well and the fracture treatment of the same well can vary from immediately thereafter to as much as 18 months depending on the availability of frac equipment which is in high demand. There are therefore several inefficiencies in the known methods of resource recovery.
It is useful to more fully discuss the conventional drilling and fracking methodology in order to assist in distinguishing the method of the present invention.
Conventional Drilling
A drill bit(s) is mounted on the end of a drill pipe, and a mixture of water and additives (“mud”) is pumped into the hole to cool the bit and flush the cuttings to the surface as the drill bit(s) grinds away at the rock. This mud generally cakes on the walls of the wellbore, which assists in keeping the well intact. The hole is generally drilled to just under the deepest fresh water reservoir near the surface, where the drill pipe is then first removed. Surface casing is then inserted into the drilled hole to a point below the water reservoir in order to isolate the fresh water zone. Cement is subsequently pumped down the casing, exits through an opening called a shoe at the bottom of the casing and wellbore, and is then forced up between the outside of the casing and the hole, effectively sealing off the wellbore from the fresh water. This cementing process prevents contamination of the freshwater aquifers. The drill pipe is then lowered back down the hole to drill through the plug and cement and continue the vertical section of the well. At a certain depth above the point where a horizontal well is desired (the “kick-off point” or “KOP”), the well will slowly begin to be drilled on a curve to the point where a horizontal section can be drilled. The KOP is often located approximately 220 meters above the planned horizontal leg. Up to this point, the process is the same as drilling a vertical well.
Once the KOP is reached, the pipe and bit are pulled out of the hole and a down hole drilling motor with measurement drilling instruments is lowered back into the hole to begin the angle building process. In general, it takes approximately 350 m of drilling to make the curve from the KOP to where the wellbore becomes horizontal (assuming an 8° angle building process, for instance). Then, drilling begins on the “lateral”, the well's horizontal section.
When the targeted horizontal drilling distance is reached on the lateral, the drill bit and pipe are removed from the wellbore. Production casing is then inserted into the full length of the wellbore. Cement is again pumped down the casing and out through the hole in the casing shoe, forcing the cement up between the outside of the casing and the wall of the hole, thus filling the “annulus”, or open space. At this point, the drilling rig is no longer needed so this equipment is moved off-site and a well head is installed. The fracturing or service crew then moves its equipment on-site to prepare the well for production and the recovery of hydrocarbon and gas resources.
Conventional Perfing and Fracking of the Wellbore
The first step in the known method is to perf the casing. In this respect, a perforating gun is lowered by wire line into the casing to the targeted section of the horizontal leg (i.e. in general, to the end of the lateral so that the process can work back along the horizontal leg from the “toe” to the “heel” of the wellbore). An electrical current is sent down the wire line to the perf gun, which sets off a charge that shoots small evenly-spaced holes through the casing and cement and out a short distance into the rock formation (often shale). This causes fractures in the rock formation, but is generally not sufficient in itself to create proper fairways through which hydrocarbons or gas can readily flow into the wellbore due to the tight or compressed nature of the rock formation (as previously stated, compressed reservoirs do not generally contain natural fractures and therefore hydrocarbons or gas cannot be produced economically without additional manipulation). As a result, a further step is needed to increase the porosity and permeability of the rock by providing more significant pathways through which the hydrocarbons or gas can flow more readily. To do this, the perf gun is removed from the hole, and the well then needs to be “fracked” to create proper fairways.
Fracking (or fracing) is the process of propagating the fracture in the rock layer caused by the perforation in the formation from the perf gun. In this respect, it is hydraulic fracturing that is usually undertaken, which is the process whereby a slurry of, for example, mainly water, and some sand and additives are pumped into the wellbore and down the casing under extremely high pressure to break the rock and propagate the fractures (sufficient enough to exceed the fracture gradient of the rock). In particular, as this mixture is forced out through the vertical perforations caused by the perf gun and into the surrounding rock, the pressure causes the rock to fracture. Such fracturing creates a fairway, often a tree-like dendritic fairway, that connects the reservoir to the well and allows the released hydrocarbons or gas to flow much more readily to the wellbore. Once the injection has stopped, often a solid proppant (e.g. silica sand, resin-coated sand, man-made ceramics) is added to the fluid and injected to keep the fractures open. The propped fractures are permeable enough to allow the flow of hydrocarbons or gas to the well.
In order for the next section of the horizontal leg to be perforated and fracked (i.e. multi-stage fracking from the “toe” all along to the “heel” of the horizontal leg), a temporary plug is placed at the nearest end of the first-stage frac to close off and isolate the already perforated and fracked section of the wellbore. The process of perfing, fracking, and plugging is then repeated numerous times until the entire horizontal distance of the wellbore is covered. Once such a process has been completed, the plugs are drilled out, allowing the hydrocarbons or gas to flow up the wellbore to a permanent wellhead for storage and distribution. Unfortunately, in this known method, a well operator is unable to determine whether any particular fracture treatment has been successful in increasing the porosity and permeability of the rock formation at a given location of the wellbore, whether the treatment is having a net positive or negative effect on overall flow of hydrocarbons or gas into the well, and whether a modification to the fracturing fluid/slurry, for example, would have produced better results.
Persons skilled in the art would be aware of other similar or related completion methodologies that have the same limitations. For instance, engineers may employ an open hole completion where no casing is cemented in place across the horizontal production leg. Pre-holed or slotted liners/casing may be employed across the production zone. Swellable/inflatable elastomer packers may be used, for instance, to provide zonal isolation and segregation, and zonal flow control of hydrocarbons or gas. Perfing may be accomplished by perforating tools or by a multiple sliding sleeve assembly, etc. Regardless, the methodologies operate in essentially the same manner—the operation proceeds from the “toe” of the well back to the “heel”, and the well operator is unable to determine whether any particular fracture treatment has been successful in increasing the porosity and permeability of the rock formation at any given location of the wellbore, whether the treatment is having a net positive or negative effect on overall flow of hydrocarbons or gas into the well, and whether a modification to the fracturing fluid/slurry, for example, would have produced better results.
A method that would allow for the creation of fracture treatments into a wellbore while the drilling operation is under way would overcome several problems and inefficiencies associated with the known hydrocarbon and gas recovery process in the oil and gas industries.